This project focuses on the transcription factors HB9 and BSAP, originally identified in B lymphocytes. The HB9 gene also referred to as HlxB9 encodes a homeodomain coding protein. Gene targeting in mice has revealed a critical role for HB9 in motor neuron and pancreas development. Abnormalities in the sacral region occurs in humans with one abnormal HB9 allele. Mice heterozygotic for an HB9 null mutation are normal, but mice that lack HB9 die at birth of respiratory failure due to a lack of diaphragm innervation. The motor neuron defects may arise from the inappropriate expression of genes normally expressed in other types of neurons. These mice also lack a portion of their pancreas, the dorsal lobe, and have reduced numbers of insulin producing beta cells in their residual ventral lobe. Because the developmental defects and perinatal lethality makes assesmment of the function of HB9 in adult tissues impossible, we have used a conditional gene targeting approach and have introduced loxP sites on either side of exon 3 of the HB9 gene. We have shown that the introduction of the LoxP sites does not interfere with normal HB9 expression. We have obtained MX-Cre transgenic mice, where the Cre recombinase can be induced by interferon treatment. We are now introducing the MX-Cre transgene onto the HB9 loxP background. The HB9 mutation has been back crossed on to a C57Black/6 background. We have reconstituted lethally irradiated Rag -/- C57Black/6 mice with with fetal liver cells prepared from HB9 -/- deficient embryos; however, while we noted some minor abnormalities in the lymphocyte phenotypes, we failed to find any major changes in lymphopoiesis or lymphocyte function. To identify HB9 target genes a genomic array approach has been used. Chips containing oligonucleotides from 15,000 different genes have been analyzed using mRNA prepared from wild type and HB9 deficient embryos. We are now verifying a panel of changes in gene expression changes by in situ hybridization and immunohisto-chemistry. BSAP is a transcription factor critical for B-lymphocyte development and lineage committment. Mice overexpressing BSAP have B cells which are hyperproliferative, have an apoptosis defect, and a propensity for developing lymphomas. Over the past year we have documented the frequency of lymphomas in these mice. We have also shown that the introduction of another transgene for a mutated human c-myc gene greately accelerates the onset of lymphoma and alters the phenotype such that it ressemble a human lymphoma. We are currently using genomic arrays to analyze gene expression in B cells from the BSAP transgenic and the double transgenic mice.